Aluminium alloy

ABSTRACT

In a first aspect, the invention provides aluminium alloy comprising the following composition, all values in weight %: Si 0.25-1.5 Cu 0.3-1.5 Fe up to 0.5 Mn up to 0.1 all other elements including Mg being incidental and present (if at all) then in an amount less than or equal to 0.05 individually, and less than or equal to 0.15 in aggregate, the balance being aluminium. In a second aspect, the invention provides a composite aluminium sheet product comprising a core layer and at least one clad layer wherein the at least one clad layer is an aluminium alloy comprising the following composition, all values in weight %: Si 0.25-1.5 Cu 0.3-1.5 Fe up to 0.5 Mn up to 0.1 all other elements including Mg being incidental and present (if at all) then in an amount less than or equal to 0.05 individually, and less than or equal to 0.15 in aggregate, the balance being aluminium. In a third aspect, the invention provides a method of making a joined structure of a steel component and an aluminium component made from the alloy and/or the sheet product of the invention.

This invention concerns an aluminium alloy and sheet alloy productprimarily intended for use in transportation vehicles. The aluminiumalloy is based on the Al—Si—Cu system and is particularly suited for useas a sheet product useful in the manufacture of automobiles. Thealuminium alloy is also suitable for use as a clad layer on a compositesheet. The invention also concerns a joined structure comprising a steelcomponent and an aluminium component.

The use of aluminium alloys in the production of automobiles and othertransportation vehicles has been established for many years. A range ofdifferent alloys are used depending on the particular requirements ofspecific components. In certain applications it is desirable that thematerial be of high strength. Yet other applications require higherformability and, in such cases, strength may be considered lessimportant. There has also been a desire for materials that deform easilyunder impact, for example in the event of collision with pedestrians andsuch materials may have even lower strengths. Aluminium alloy productsfor such applications are provided in various forms, from sheet toforgings, extrusions to castings.

Typically the aluminium alloys are from the 6XXX series of alloys, whoseprincipal alloying elements are Mg and Si, or from the 5XXX series ofalloys, where the principal alloying element is Mg. There has beenoccasional use of the 2XXX series alloys where the principal alloyingelement is Cu. For an understanding of the number designation systemmost commonly used in naming and identifying aluminium and its alloyssee “International Alloy Designations and Chemical Composition Limitsfor Wrought Aluminum and Wrought Aluminum Alloys”, published by TheAluminum Association, revised February 2009.

Clad sheets or composite sheets are also known for use in automotive andother applications. In such products the composite sheet consists of atleast two layers of alloys with different chemical compositions. Onelayer, typically called the core, provides bulk mechanical properties,whilst the second layer, typically called the clad layer, providesspecific surface characteristics. The clad layer is usually thinner thanthe core layer. Commonly the core layer of one composition is interposedbetween two clad layers of another composition to form a three-layersheet, both clad layers having the same composition. But this is notalways the case and a composite sheet may be provided of multiplelayers, each layer having a different composition.

Aluminium alloys are not the only materials used in construction oftransportation vehicles; steel remains an important structural material.Whilst concerned primarily with automotive structures, the inventiondescribed herein is equally applicable to other transportation vehiclesincluding but not limited to aircraft and land vehicles such as trains,buses and trucks as well as other industrial applications where there isa need to join aluminium components to steel components. The case ofautomotive structures is used to illustrate the background to theinvention and to demonstrate its benefits.

At various locations within the automobile structure, the aluminiumalloy must come into contact and be joined to a steel alloy product.This creates problems because aluminium and steel cannot besatisfactorily joined by conventional welding techniques, such as TIG,MIG, laser welding, plasma welding, etc., due to a large differencebetween liquidus temperatures and low inter-element solubility. Indeed,classically defined welding, in the sense of coalescence of two moltenmetals, does not occur because the temperatures used are generally nothigh enough to cause the steel to melt. Various terms are used,therefore, to describe the thermal joining process that takes place andsuch terms may include but are not limited to laser welding, brazewelding and so on. In essence, and for the purposes of this invention, astructure that comprises an aluminium part joined to a steel part meansone that arises from a thermal process that causes at least a part ofthe aluminium component to melt.

The binary Al—Fe equilibrium phase diagram indicates that variousequilibrium intermetallic compounds such as Fe₂Al₅, FeAl₃, FeAl₂ andFeAl exist. These intermetallic compounds are known to be hard andbrittle. In addition, the high heat input of conventional weldingtechniques and the resulting reaction and diffusion between the steeland aluminium parts can give rise to a thick layer of brittleintermetallics. The presence of such intermetallics at steel/aluminiuminterfaces may lead to poor mechanical properties and brittle fracturebehavior of the joint. The joint between the aluminium and steel alloyscan thus become a site of key structural weakness. A joint that hasreasonable fracture strength, one that possesses sufficient ductility,is preferable.

Attempts have been made to improve the interfacial strength andductility of such joints. One approach has been to reduce the heat inputto the joining process by, for example, increasing the welding speed,adding a backing block to extract heat or interrupting the weldingprocess. Such an approach is embodied within the known technique of ColdMetal Transfer braze-welding, (CMT). Disadvantages of this approach arethat, with it, manufacturing is more complicated and more expensive,there is a reduced operating window that does not lend itself to massproduction on an industrial scale and, although there is an improvementin interface strength, the fracture mode remains brittle.

A second approach to improve weldability has been to add Zn to the weldto promote formation of an Al—Zn, low melting point, eutectic structure.In this approach a Zn filler material is used without flux during thewelding operation in an air atmosphere or a Zn cladding is used on thesteel component. A low heat input may also be used in combination. Aproblem with the use of Zn is that it tends to evaporate during laserwelding. Further, Zn reduces the corrosion resistance of the jointregion because it has a highly negative corrosion potential.

JP04768487B2 describes a method for obtaining a composite structure ofaluminium and steel for motor vehicles which involves melting analuminium layer of AA5182 alloy on a steel plate using a laser beamwithout flux.

U.S. Pat. No. 4,814,022 describes a weldable aluminum alloy comprisingSi and Mg defined by a trapezium having co-ordinates at; Si 0.5, Mg 0.1;Si 0.5 Mg 0.2; Si 1.3, Mg 0.5; Si1.3, Mg 0.1. The alloy further containsCu between 0.1 and 0.5. The composition is controlled to limitprecipitation of Mg₂Si during solidification after casting and the Mg₂Siprecipitates developed in the alloy, and necessary for strengthening,arise from subsequent heat treatments. Although described as a weldablealloy, the examples describe the alloy being welded to itself, not to asteel component.

U.S. Pat. No. 4,808,247 describes a process of making Al—Si—Cu—Mg alloysthat involves the application of a final annealing step wherein thealloys described are heated to between 60-360° C., held at thattemperature for a period, and cooled in a controlled manner. Threealloys are described, all of which contain Mg to promote the formationof Mg₂Si strengthening precipitates.

U.S. Pat. No. 5,582,660 describes an alloy for use in automotive sheetcomprising the following composition; Si>1.0 to about 1.3, Mg>0.25 toabout 0.60, Cu about 0.5 to about 1.8, Mn about 0.01 to about 0.1, Feabout 0.01 to about 0.2, balance being substantially aluminium andincidental elements and impurities. The presence of Mg in combinationwith Si is essential for the formation of Mg₂Si strengtheningprecipitates.

WO 98/14626 describes an aluminium alloy for rolled products with thefollowing composition in wt % of: Si 0.8-1.5; Mg 0.2-0.7; Fe 0.2-0.7; Mn0.01-0.1; Cu up to 0.25; Cr up to 0.1; Zn up to 0.4; V up to 0.2 mbalance being Al. Silicon and Magnesium are added for the formation ofstrengthening Mg2Si precipitates. Fe is employed to form a sufficientvolume fraction of Al—Fe phases that can act as recrystallizationnucleation sites after being broken up and dispersed during rolling.

Investigators have also considered the use of Al—Si solder alloys, asevidenced by the article “The Characterisation of the IntermetallicFe—Al Layer of Steel-Aluminium Weldings” by Potesser et al, published inthe EPD Congress, 2006.

U.S. Pat. No. 7,943,883 describes a method for joining an iron memberand an aluminium member, where the iron member includes a plated layerat least on the joining side and the aluminium layer is formed by analuminium core and an aluminium cladding with a melting point lower thanthat of the aluminium core material, provided on the joining side of thewith the iron member. The alloy of the aluminium cladding layer iseither an Al—Si alloy with 4.0-11.6 wt % Si, balance Al or an Al—Cualloy with 5.7-33.2 wt % Cu, balance Al.

Further recommendations have been to reduce the thickness of theinterface zone created when joining aluminium to steel but this requiresvery tight process window control during welding and is extremelydifficult to achieve on a production scale.

These proposals still leave something to be desired, in the quality of ajoint between an aluminium alloy product and a steel alloy product.

It is an object of the invention to provide an aluminium alloy which canbe welded to a steel alloy without the use of a filler alloy and whichprovides an interface possessing a reasonable strength and a ductilefracture mechanism, a useful way to enrich the range of availablecompromises between strength and ductility.

In accordance with a first aspect of the invention an aluminium alloy isprovided comprising the following composition, all values in weight %:

Si 0.25-1.5 Cu  0.3-1.5 Fe up to 0.5 Mn up to 0.1

-   -   all other elements including Mg being incidental and present (if        at all) then in an amount less than or equal to 0.05        individually, and less than or equal to 0.15 in aggregate,    -   the balance being aluminium.

In embodiments, the amount of Mn is incidental, that is, not more than0.05 wt %. In practice an amount of 0.08 wt % for Mn is contemplated,but this for expedience, and commercial practicality, rather than toenhance performance in meeting technical targets.

In contrast to the prior art solutions to the technical problem, theinventors have adapted the aluminium alloy composition to improve thewettability of the aluminium alloy, to reduce the susceptibility of thealloy to hot cracking shortness, to modify the diffusion of Fe from thesteel into the aluminium alloy product and to bias the type ofintermetallic formed close to the steel to favour the FeAl type over theFeAl₃-type. With alloys according to the invention, the interface ischaracterized by a dense intermetallic layer comprising twointermetallic types, FeAl and Fe₂Al₅, with FeAl in the zone adjacent thesteel alloy. In addition, the interface region created with the alloy ofthe invention is relatively large, comprising 3 distinct zones. Thisthicker interface zone permits the use of wider processing parameters,giving greater process flexibility and thereby rendering the new alloysuitable for large scale industrial production.

Si is added to the alloy to reduce the solidus temperature and toimprove the wettability. For these reasons the lower limit of Si is setat 0.25. Further, additions of Si help reduce the susceptibility of hotcracks forming after welding and a preferred lower limit for Si is 0.5.The upper limit of Si is set to 1.5 because a higher Si level favoursthe formation of Al(Fe3,Si)-type intermetallics and has a negativeeffect on ductility and the preferred upper limit of Si is 1.25.

Cu is also added to the alloy to reduce the solidus temperature and toimprove the wettability but it is also added to modify the Al—Feintermetallic type. For these reasons the lower limit of the Cu contentis set at 0.3. The amount of Cu should not be too high, however, becausea higher Cu content increases the risk of hot cracking. Further, higherCu contents also reduce the joint ductility. For these reasons the upperlimit of Cu is set at 1.5 although in some situations setting an upperlimit for Cu of 1.25 may be desirable.

Whilst Mg in combination with Si would lead to the formation ofstrengthening Mg₂Si precipitates, that is not helpful here because Mgdoes not contribute to the improvement of the joint quality. As the Mgcontent is increased there are declines in the ductility of the joint,in the formability of the alloy and in the quality of the weld, porosityand cracking. A small amount of Mg may be tolerated, (to accommodatescrap recycling), but the Mg content should not exceed that of anindividual impurity element.

Mn also makes no significant impact on the hot cracking susceptibilityor formability but it may be present in recycled metal from othersources. Here it can be tolerated in amounts higher than would be thecase for other elements without serious adverse effect. Thus, forcommercial reasons (more recycling) an amount higher than that permittedfor other incidental impurity elements is permitted in the case of Mn

Other elements such as, but not limited to, Zn, Ni, Ti, B, Cr and V maybe present in the form of trace elements or unavoidable impurities or,in the case of Ti and B, through the addition of grain refiners. Eachsuch trace element or unavoidable impurity or grain refining element ispresent in an amount less than 0.05 each and less than 0.15 in total.The balance of the alloy is aluminium.

In accordance with a second aspect of the invention a compositealuminium sheet is provided, said composite aluminium alloy sheetcomprising a core and at least one clad layer wherein the clad layercomprises the following composition, all values in weight %:

Si 0.25-1.5 Cu  0.3-1.5 Fe up to 0.5 Mn up to 0.1

-   -   all other elements including Mg being incidental and present (if        at all) then in an amount less than or equal to 0.05 each or        less than or equal to 0.15 in aggregate    -   the balance being aluminium.

In embodiments, the amount of Mn is incidental, that is, not more than0.05 wt %. In practice an amount of 0.08 wt % for Mn is contemplated,but this for expedience, and commercial practicality, rather than toenhance performance in meeting technical targets.

In the context of composite sheets, the term “core” layer is used toindicate the alloy contributing most to the bulk properties of thecomposite sheet and the term “clad” is used to indicate the alloy at thesurface providing surface properties for the composite sheet. Compositesheets may comprise a single clad layer on a single core layer althoughmore often they comprise two clad layers on either side of the singlecore layer. Typically the clad layers are thinner than the core layer,on their own and as a combined total.

Where the alloy is used as a clad layer on a composite sheet, the corelayer may be a 6XXX series alloy or a 5XXX series alloy as understood byreference to the Aluminum Association Teal Sheets. If the core layer isa 6XXX series alloy it may be selected from the group consisting ofAA6016, AA6016A, AA6014, AA6011, AA6111, AA6009, AA6010, AA6022 andAA6451. If the core alloy is a 5XXX series alloy it may be selected fromthe group consisting of AA5005, AA5152, AA5052, AA5018, AA5454, AA5754,AA5056, AA 5456, AA5182, AA5186, AA5059, AA5083 and AA5383. An advantageof using the new alloy in a composite sheet, wherein the core is a highstrength alloy, is that the entire sheet is far less susceptible todistortion during further processing of the vehicle body such as, forexample, during the thermal treatment of paint baking.

In accordance with a third aspect of the invention a joined structure isprovided wherein the joined structure comprises a steel component and analuminium alloy component joined thereto and wherein the aluminium alloycomponent is made from an aluminium alloy comprising the followingcomposition, all values in weight %:

Si 0.25-1.5 Cu  0.3-1.5 Fe up to 0.5 Mn up to 0.1

-   -   all other elements including Mg being incidental and present (if        at all) then in an amount less than or equal to 0.05 each or        less than or equal to 0.15 in aggregate    -   the balance being aluminium.

In embodiments, the amount of Mn is incidental, that is, not more than0.05 wt %. In practice an amount of 0.08 wt % for Mn is contemplated,but this for expedience, and commercial practicality, rather than toenhance performance in meeting technical targets.

In accordance with a fourth aspect of the invention a joined structureis provided wherein the joined structure comprises a steel component andan aluminium alloy component joined thereto and wherein the aluminiumalloy component is made from a composite aluminium alloy sheetcomprising a core and at least one clad layer wherein the clad layercomprises the following composition, all values in weight %:

Si 0.25-1.5 Cu  0.3-1.5 Fe up to 0.5 Mn up to 0.1

-   -   all other elements including Mg being incidental and present (if        at all) then in an amount less than or equal to 0.05 each or        less than or equal to 0.15 in aggregate    -   the balance being aluminium.

In embodiments, the amount of Mn is incidental, that is, not more than0.05 wt %. In practice an amount of 0.08 wt % for Mn is contemplated,but this for expedience, and commercial practicality, rather than toenhance performance in meeting technical targets.

For the purpose of this invention the word “joined” is intended to meana joint resulting from a thermal process operating at a temperature thatcauses melting of at least a part of the inventive alloy component. Thethermal process used does not lead to melting of the steel component.Therefore “welding”, in the classic sense of coalescence of two or moremolten metals, does not occur. Since the use of a flux is not necessary,(although it could be used), the process is not classical brazingalthough one can describe the process as fluxless brazing. Others haveused the term “braze-welding”. Under the application of sufficient heat,most conveniently from a laser but conceivably from other sources, thealloy of the aluminium component melts and reacts with the surfacelayers of the steel component, including the zinc coating, if such acoating is present. The temperature is sufficiently high that diffusionof Fe from the steel component into the molten aluminium occurs and,when the molten aluminium cools and freezes, a series of layers rich inintermetallic compounds is formed with the Al/Fe ratio increasing as thedistance from the steel component increases. A Zn coating on the steelcomponent improves the wettability of the aluminium alloy of theinvention and it is preferred that the steel component be provided withsuch a Zn layer.

According to a fifth aspect of the invention there is provided a methodof making a joined structure wherein the joined structure comprises asteel component and an aluminium alloy component and wherein the steeland aluminium alloy components are joined by a thermal process thatcauses at least a part of the aluminium component to melt and whereinthe aluminium alloy component is made from an alloy that has thefollowing composition:

Si 0.25-1.5 Cu  0.3-1.5 Fe up to 0.5 Mn up to 0.1

-   -   all other elements including Mg being incidental and present (if        at all) then in an amount less than or equal to 0.05 each or        less than or equal to 0.15 in aggregate    -   the balance being aluminium.

In embodiments, the amount of Mn is incidental, that is, not more than0.05 wt %. In practice an amount of 0.08 wt % for Mn is contemplated,but this for expedience, and commercial practicality, rather than toenhance performance in meeting technical targets.

In comparison to the prior art, the thermal joining of a steel componentwith said aluminium alloy, yields improved hot crack-resistance,ductility and strength of the joint.

At the interface region between the steel sheet and the aluminium sheet,the bias towards particles of FeAl over other species of Fe/Alintermetallics helps to render the interface less brittle and moreductile without any unacceptable loss of strength.

The aluminium alloy according to the invention is intended for primaryuse in sheet form but the scope of the invention is not limited to thatform. The skilled person will understand that the alloy of the inventioncan be provided in other product forms, such as extrusions, and canstill be welded to steel components. Although the primary focus is onautomotive structures, the skilled reader will realize that the alloy ofthe invention, and its use in joined structures incorporating steel, canbe applicable to many different applications in the transportationsector, (marine, rail, aerospace), as well as many other industrialapplications, (construction, plant machinery, etc.).

In the following, the invention will be described in more detail byreferring to examples and Figures which show the results of testsconducted on embodiments of the claimed invention. Neither the detaileddescription nor the Figures are intended to limit the scope ofprotection which is defined by the appended claims.

FIG. 1 is a plot of a stress-displacement curve for an alloy accordingto the invention.

FIG. 2 is a plot of the effect of Cu on the equilibrium solidus andliquidus temperature.

FIG. 3 is a plot of the effect of Cu on calculated hot-crackingsusceptibility.

FIG. 4 is a plot of the effect of Cu on joint ductility.

FIG. 5 is a plot of the effect of Cu on joint strength.

FIG. 6 is a plot of the effect of Si on the equilibrium solidus andliquidus temperature

FIG. 7 is a plot of the effect of Si on joint ductility.

FIG. 8 is a plot of the effect of Si on joint strength.

FIG. 9 is a plot of the effect of Mg on bending and elongation.

FIG. 10 is a plot of the effect of Mg on weld quality.

FIG. 11 is a plot of the effect of Mg on joint ductility.

FIG. 12 shows two images of the interface produced when an AlSi10 alloyis welded to steel sheet including phase analysis

FIG. 13 shows two images of the interface produced when an alloyaccording to the invention is welded to steel sheet including phaseanalysis

FIG. 14 is a plot of the stress-displacement curves for two compositesheets after joining to steel, one according to the invention andanother according to the prior art,

EXAMPLE 1

Table 1 lists the compositions of alloys cast in the form of smallingots, each ingot measuring 20×150×200 mm.

TABLE 1 Sample Si Fe Cu Mn Mg 1 0.50 0.30 0.46 <0.01 <0.01 2 0.51 0.191.02 <0.01 <0.01 3 0.51 0.31 1.48 <0.01 <0.01 4 0.005 0.20 0.99 <0.01<0.01 5 0.98 0.20 1.02 <0.01 <0.01 6 1.48 0.20 0.98 <0.01 <0.01 7 2.970.20 1.00 <0.01 <0.01 8 0.51 0.20 0.98 <0.01 0.26 9 0.51 0.20 0.99 <0.010.50 10 0.51 0.21 1.02 <0.01 2.00

All alloys contained less than 0.05 of other elements individually andless than 0.15 in total, the balance being aluminium. Samples 8, 9 and10 are comparative, and not within the scope of the claims below.

The ingots were homogenized in an air furnace at 550° C. for 6 hours,hot-rolled to 10 mm and cold rolled to 1 mm. The sheet samples wereannealed at 430° C. for 1 hour to cause recrystallization. A finalleveling operation was applied to the 1 mm sheet.

Sheet samples were then joined by a fluxless laser welding process to a1 mm sheet of low-alloyed steel coated with a 7 μm zinc layer (hot dipgalvanized) using an Nd-YAG laser with a constant power of 3 kW. Thejoining geometry was flange welding (Kehlnaht) with a laser angle of 60°and no gap between the two sheets. The laser speed was 4 m/min for allalloy combinations.

The compositional effect of the different elements on the equilibriumsolidus and liquidus temperatures was calculated using commercialthermodynamic software from JMatPro coupled to in-house database. Thehot cracking susceptibility was also calculated on the basis ofthermodynamics calculation of the solid fraction evolution through thesolidification interval. In both cases, nominal alloy compositions wereused.

All samples of joined sheets were subjected to dye penetrant inspection(DPI) to assess the visual integrity of the joints. The quality of thejoint under DPI was based on a simple ranking system from 1 to 4, with 1being good, 4 being bad (containing a large number of hot-cracks or/andcoarse porosity).

The nature and distribution of intermetallics produced in the interfacezone was evaluated by conventional SEM and EDX analysis.

The joined samples were also subjected to lap shear tensile testing toassess joint fracture strength and ductility. It is not appropriate touse conventional stress-strain curves in such figures because the testconfiguration means that the tensile stress, and thus plasticdeformation, is not constant throughout the specimen. The results oftensile tests on lap shear joints are presented as equivalent stress inthe aluminium section against grip-to-grip distance during the test,(described herein as standard travel). The equivalent stress within thealuminium part of the joined sample is the nominal force divided by thecross-sectional area of the aluminium section. The standard travel is anindication of the ductility of the joint.

Some samples were subjected to 3-point bending tests to evaluateformability. The formability of the samples was measured using a bendtest based on DIN 50111, but with slight modifications to the procedure.In this test a 60 mm×60 mm piece of sheet, with a prior pre-straining of10% (uniaxial stretching), was placed over two cylindrical rolls, therolls being separated by a distance equal to twice the sheet thickness.Each roll diameter was 30 mm. Under load, a tapered punch bar of width100 mm pushes the sheet into the gap between the rolls. The punch forceis measured as well as the displacement. At the point of plasticdeformation, (i.e. the start of cracking), the load necessary to deformthe sheet falls, the punch force reduces and the test is automaticallystopped. The sheet thus tested is deformed into a V shape and theinternal angle of the V is measured. In this test a lower angletranslates into better formability of the sheet. This test, (hereinafterreferred to as “the modified DIN 50111 test”), is preferable to otherformability tests because the results do not depend so much, if at all,on operator judgement.

Samples 1-3 illustrate the effect of Cu on the performance of thealloys. Samples 2 with 5-7 illustrate the effect of Si on performance.Samples 2 with 8-10 illustrate the effect of Mg on performance.

FIG. 1 shows the stress-displacement curve for sample 2 after joining.The standard travel of the test piece, proportional to elongation isvery high, indicating a ductile fracture mode which was also apparent inthe fracture surface.

Effect of Cu. FIG. 2 shows the effect of increasing Cu content to a basecomposition of Al0.5Si on the solidus of the alloys. Adding Cu reducesthe solidus temperature and improves wettability. FIG. 3 shows theeffect of Cu on hot-cracking susceptibility with hot-cracking morelikely as the Cu content increases up to 1.5%. FIG. 4 shows the effectof Cu on joint ductility. FIG. 5 shows the effect of Cu on the jointfracture strength. Increasing Cu from 0.5 to 1.0% increases fracturestrength but it falls again slightly if the Cu content is increasedtowards 1.5%. From FIGS. 3, 4 and 5 we can see that the Cu contentshould be not be >1.5% and is preferably up to 1.25%.

Effect of Si. FIG. 6 shows the effect of increasing Si content to a basecomposition of Al1.0Cu on the solidus of the alloys. Adding Si reducesthe solidus temperature and improves wettability. FIG. 7 shows theeffect of Si on joint ductility. Increasing Si content up to 1.0%improves bond ductility but there is a rapid decline in bond ductilityas the Si content increases to 1.5% and beyond. FIG. 8 shows thatincreasing the Si content leads to an increase in joint fracturestrength up to a 1% addition but the fracture strength declines as moreSi is added. From FIGS. 7 and 8, we can see that Si should be limited tono more than 1.5% and preferably no more than 1.25% to maintain goodjoint qualities in terms of ductility and fracture strength.

Effect of Mg. FIG. 9 shows the effect of Mg content on bendability asmeasured using the modified DIN 50111 test. The effect on elongation isminimal. As the Mg content increases, the bendability of samplesprestrained by 10% diminishes towards an Mg content of 0.5 but thenimproves again as the Mg content is raised further to 2%. FIG. 10 showsthe effect of Mg content on visual weld quality after DPI. Additions ofMg from essentially no Mg to 0.5 Mg led to worse weld quality, (coarseporosity and the presence of weld cracks), but the weld quality improvedagain when 2% Mg was added. The effect of Mg on weld ductility is shownin FIG. 11 and increased Mg content lowers weld ductility. For thesereasons the Mg content is limited to the amount of an incidental elementor impurity.

FIGS. 12, a) and b), show SEM images of the interface seen with AlSi10alloys (sample 0) joined to steel. In the interface produced with AlSi10alloys the width of the interface is approximately 10 μm and the regionimmediately next to the steel alloy comprises an intermetallic zonedominated by FeAl₃ (high Al/Fe ratio, in atomic %). The brittlestructure is evidenced by the high amount of micro-cracks in the layer.FIGS. 13, a) and b), show SEM images and EDX spectra of the interfaceproduced when sample 2 was joined to steel. The width of the interfaceis approximately 20 μm and the image reveals a dense and crack-freeintermetallic layer. EDX analysis clearly shows that the continuousintermetallic layer at the interface is composed of two phases withvarious Al/Fe ratios. A third region on the top of the layer, withintermetallics in the shape of needles and a higher Al/Fe ratio ispresent. The first two intermetallic types are close to the FeAl andFe₂Al₅ stoichiometry, whereas the third type is close to the morebrittle FeAl₃. There are fundamental differences between the interfacesincluding the presence of an FeAl-type layer adjacent the steelcomponent when the steel component is joined to the inventive alloy.

EXAMPLE 2

Two composite sheet products were produced where the “core” layer was anAA6016 alloy and a single “clad” layer was applied of either an Al—Sialloy typical of the prior art (Sample 11) or an Al—Cu—Si alloy of thepresent invention (Sample 12). The clad layers in each sample accountedfor 10%, (+/−1%), of the total sheet thickness. The alloy compositionsof each layer are shown in Table 2.

TABLE 2 Sample Si Fe Cu Mn Mg Core alloy 0.61 0.18 0.15 0.05 0.67 11clad 9.91 0.11 <0.01 <0.01 <0.01 12 clad 0.51 0.17 0.98 0.06 <0.01

The ingots were homogenized in an air furnace at 550° C. for 6 hours,hot rolled to 10 mm and cold rolled to 1 mm. The sheet samples weresolution heat treated at 540° C. for 40s, rapidly cooled by air fans andthen pre-aged by holding samples at 100° C. for 1 hr.

Some samples were allowed to naturally age to the T4PX condition afterbeing subjected to a 10% pre-strain, a simulation of a typical formingoperation. Other samples were further aged to the T8X (paint-baked)condition by subjecting them to a 2% pre-strain followed by ageing at185° C. for 20 minutes and yet more samples were prepared in the T62temper by subjecting them to a heat treatment at 205° C. for 30 minutes.The mechanical properties for sample 12 in three different temperconditions are summarized in Table 3.

TABLE 3 T4PX DC bend T8X T62 Rp0.2 Rm A80 angle Rp0.2 Rm A80 Rp0.2 RmA80 MPa MPa % ° MPa MPa % MPa MPa % 112 227 25.6 15 218 281 19.1 242 28913.5

They were then joined to steel sheet under the same laser weldingconditions as described in example 1. The joined parts were mechanicallytested to evaluate the strength and ductility of the joint.

The stress-strain curve of FIG. 14 shows the results for both samples 11and 12. In the case of sample 12, the curve is for the product in theT8X condition. There is a dramatic improvement in the strength attainedand the ductility for the product according to the invention comparedwith these qualities for a sample not in accordance with the claimsbelow.

1-15. (canceled)
 16. An aluminum alloy comprising the followingcomposition, all values in weight %: Si 0.25-1.5 Cu  0.3-1.5 Fe up to0.5 Mg <0.1 Mn up to 0.2

unavoidable impurities less than or equal to 0.05 each and less than orequal to 0.15 in total, balance aluminum.
 17. The aluminum alloy ofclaim 16, wherein: Mn is up to 0.1 and Mg is in an amount less than orequal to 0.05.
 18. The aluminum alloy of claim 16, wherein the Sicontent is 0.5-1.25.
 19. The aluminum alloy of claim 16, wherein the Cucontent is 0.3-1.25.
 20. The aluminum alloy of claim 16, wherein the Mncontent is not more than 0.08.
 21. The aluminum alloy of claim 16,wherein the Mn content is <0.01.
 22. The aluminum alloy of claim 16,wherein the Mg content is <0.01.
 23. A joined structure comprising asteel component and an aluminum alloy component joined thereto andwherein the aluminum alloy component is made from the aluminum alloy ofclaim
 16. 24. The joined structure of claim 20, wherein an interfacezone between the steel component and the aluminum alloy componentcomprises an FeAl layer adjacent to the steel component.
 25. A compositealuminum sheet product comprising a core layer and at least one cladlayer wherein the at least one clad layer is an aluminum alloycomprising the following composition, all values in weight %: Si0.25-1.5 Cu  0.3-1.5 Fe up to 0.5 Mg <0.1 Mn up to 0.2

unavoidable impurities less than or equal to 0.05 each and less than orequal to 0.15 in total, balance aluminum.
 26. The composite aluminumsheet product of claim 25, wherein: Mn is up to 0.1 and Mg is in anamount less than or equal to 0.05.
 27. The composite aluminum sheetproduct of claim 25, wherein the core layer is made from an alloyselected from the group of 5XXX and 6XXX series alloys.
 28. Thecomposite aluminum sheet product of claim 25, wherein the core layer ismade from an alloy selected from the group consisting of AA6016,AA6016A, AA6014, AA6011, AA6111, AA6009, AA6010, AA6022 and AA6451. 29.The composite aluminum sheet product of claim 25, wherein the core layeris made from an alloy selected from the group consisting of AA5005,AA5152, AA5052, AA5018, AA5454, AA5754, AA5056, AA5456, AA5182, AA5186,AA5059, AA5083 and AA5383.
 30. A joined structure comprising a steelcomponent and an aluminum alloy component joined thereto and wherein thealuminum alloy component is made from the composite aluminum sheetproduct of claim
 25. 31. A method of making a joined structurecomprising a steel component and an aluminum component comprisingjoining the steel and aluminum components by a thermal process thatcauses at least a part of the aluminum component to melt, and whereinthe aluminum component is made from the alloy of claim
 16. 32. A methodof making a joined structure comprising a steel component and analuminum component comprising joining the steel and aluminum componentsby a thermal process that causes at least a part of the aluminumcomponent to melt, and wherein the aluminum component is made from thecomposite aluminum sheet product of claim
 25. 33. The method of claim31, wherein the thermal process is laser welding.
 34. The method ofclaim 32, wherein the thermal process is laser welding.
 35. The methodof claim 31, further comprising treating Fe and Mn as impurities, andreducing their presence to the extent that is practicable.
 36. Themethod of claim 32, further comprising treating Fe and Mn as impurities,and reducing their presence to the extent that is practicable.
 37. Themethod of claim 35, wherein the thermal process is laser welding. 38.The method of claim 36, wherein the thermal process is laser welding.